This invention relates generally to nuclear reactors and more particularly, to a control rod guide tube inspection camera fixture and method of inspecting control rod guide tubes in a nuclear reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
The core of the reactor includes an array of fuel bundles with square cross section. The fuel bundles are supported from below by a fuel support. Each fuel support supports a group of four fuel bundles. The heat generated in the core can be decreased by inserting control rods into the core, and the generated heat can be increased by retracting control rods from the core. In some BWR""s, the control rods have a cruciform cross section with blades that can be inserted between the fuel bundles of a group of four.
The control rods are driven by control-rod drives that extend through the bottom of the reactor vessel. The control-rod drives are supported by control-rod-drive (CRD) housings, which are tubes that extend through the vessel bottom. The CRD housings support control-rod guide tubes (CRGTs) that help guide the vertical movement of the control rods. The fuel supports are seated on the CRGTs.
Internal structures of operating BWRs are susceptible to various corrosive and cracking processes. Stress corrosion cracking (SCC) is one known phenomenon occurring in reactor components, such as structural members, piping, control rod guide tubes, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to SCC.
Internal structures of BWR""s susceptible to corrosive and cracking process are typically inspected during reactor shutdown periods. Inspections can include ultrasonic inspections and visual inspections utilizing a camera. Typically, cameras are operated by hand by workers that have descended underwater into the reactor. This method of visual inspection is time consuming because of the limited time workers can stay inside the reactor. Also, because of water flow, sometimes caused by water thermals, and the hand held method of camera usage, the quality of the camera shots can vary. Sometimes the camera shots are unacceptable, and workers must repeat the visual inspection and regenerate the unusable camera shots. Repeating portions of the inspection lengthens the down time of the reactor and causes higher maintenance costs.
In one aspect, a guide tube inspection camera fixture for inspecting internal surfaces of nuclear reactor control rod guide tubes is provided. The guide tube camera fixture includes a base plate, a unit-body tower having a first end and a second end, and a support bracket coupled to the unit-body tower. The unit-body tower is coupled to the base plate, and the support bracket is configured to support an inspection camera.
In another aspect, an inspection camera assembly for inspecting internal surfaces of nuclear reactor control rod guide tubes is provided. The inspection camera assembly includes an inspection camera coupled to a camera fixture assembly. The guide tube camera fixture includes a base plate, a unit-body tower having a first end and a second end, and a support bracket coupled to the unit-body tower. The unit-body tower is coupled to the base plate, and the support bracket is configured to support an inspection camera.
In another aspect, a method of visually inspecting internal surfaces of nuclear reactor control rod guide tubes is provided. The method includes inserting an inspection camera assembly into a guide tube to a predetermined position, and taking photographs of an inside surface of the guide tube. The inspection camera assembly includes a base plate, a unit-body tower having a first end and a second end, the unit-body tower coupled to the base plate, and a support bracket coupled to the unit-body tower, the inspection camera coupled to the support bracket.